1. Field of the Art
This invention relates to an apparatus for controlling a PWM controlled inverter which effects waveform control of the inverter output voltage in accordance with the memory contents of ROM in which instantaneous output voltage data are memorized.
2. Prior Art
For instance, in order to adjustably operate a compressor in a refrigerating cycle of an air conditioner, it is well known to carry out the adjustable speed operation of a motor for driving such a compressor through an inverter. One method of adjustably operating a motor is to use an a.c. motor as a motor to drive the a.c. motor using a PWM controlled inverter of variable frequency and variable voltage.
In a known control apparatus for controlling inverters of this kind (e.g., see U.S. Pat. No. 4,356,544), control data for PWM (Pulse Width Modulation) control every respective frequencies are memorized or stored in a ROM (Read Only Memory) to read the content of the ROM in accordance with the frequency set data to deliver a control current based therein to respective components, e.g., transistors of the inverter through a waveform shaping circuit and a base drive circuit, thus allowing them to output a predetermined a.c. voltage, e.g., a three-phase a.c. voltage.
In the case of the three-phase inverter, control data of three-phases in a section of an electrical angle of 0 to 30 degrees with an arbitrary phase being as a reference, e.g., control data in a section of 0 to 30 degrees of R-phase, in a section of -120 to -90 degrees of S-phase, and in a section of 120 to 150 degrees of T-phase with the R-phase being as a reference are memorized in a ROM along with the frequency data. The control data read from the ROM are converted to PWM control waveform corresponding to a section ranging over an electrical angle of 360 degrees by the waveform shaping circuit by making use of symmetry of waveforms in respective sections of 30 degrees. Thus, the required memory capacity of the ROM can be reduced without lowering the output waveform accuracy.
However, the control apparatus of this system still has the following inconvenience. Namely, in the PWM control system, according as the pulse generation electrical angle is finely divided, data of the same turn-on pattern occur over a plurality of electrical angle addresses, with the result that a wastefullness such that the memory data are the same, but that only addresses are different occurs, leading to the decreased utilization factor of the ROM.